Production of extruded metal products



1, 1964 A. c. BRIDGE 3,144,132

PRODUCTION OF EXTRUDED METAL PRODUCTS i Filed Dec. 5, 1955 7 Sheets-Sheet l E STRES 6 6E STRAIN 5, in

ATTORNEYS Aug. 11, 1964 A. C. BRIDGE PRODUCTION OF EXTRUDED METAL PRODUCTS Filed Dec. 5, 1955 STRESS:

LOAD AREA 7 Sheets-Sheet 2 STRA/N IN IN.

I I I l I I I I I I I I l I I STR A IN 6 6' MAX 1 I? a g. n|"

0' 5 m l 6) cow, mew sum/u RATE new mnvr {9' M 5 STRAL z i 0 I I l8 Her, mew smm/v mm! I l I 1 I I I uvvsuron fiailafi 6.322%

BY 5%; #WW

Aug. 11, 1964 A. c. BRIDGE 3,144,132

PRODUCTION OF EXTRUDED METAL PRODUCTS 7 Sheets-Sheet 3 Q 3 Q Q E Q" & 5e 8 5 II M x g $MA6IVES/l/Mg :2 8 II a y MAGNESIUM TITANIUM WRDUEHT MON E L Q ALUMINUM 5 UIMIIVUM ALUMINUM TIN 9: INVENTOR ATTORNEYS 11, 1964 A. c. BRIDGE 3,144,132

PRODUCTION OF EXTRUDED METAL PRODUCTS Filed Dec. 5, 1955 7 Sheets-Sheet 4 Aug. 11, 1964 A. c. BRIDGE 3,144,132

PRODUCTION OF EXTRUDED METAL PRODUCTS Filed Dec. 5, 1955 7 Sheets-Sheet 5 J FRACTUBE nv mus/01v TRUE STRESS STRESS 7p I w COMPRESSION 4 musfsq. IN. l I I T I l l l e: STRAIN l/V/IN. 6 MAX.

i I I i n VELOCITY INVENTOR Jrv/zz'baid C372? BY Y W lrf' ATTORNEYS Aug. 11, 1964 A. c. BRIDGE PRODUCTION OF EXTRUDED METAL PRODUCTS 7 Sheets-Sheet 6 Filed Dec. 5, 1955 3/4 Extruded [/2 Extruded H4 Extruded INVENTOR Archibahl aflmyg ATTDRN EYS Aug. 11, 1964 A. c. BRIDGE 3,144,132

PRODUCTION OF EXTRUDED METAL PRODUCTS Filed Dec. 5; 1955 7 Sheets-Sheet 'r T =.36 sec. T =.42sec.

Tl 68 sec.

1 r v lN VENTOR ATTO RNEYS United States Patent 3,144,132 PRODUCTION OF EXTRUDED METAL PRODUCTS Archibald Claude Bridge, Colwick, n'ear Nottingham,

England, assiguor, by mesne assignments, to Anglo- American Extrusion Company, a corporation of Delaware Filed Dec. 5, 1955, Ser. No. 551,153 Claims priority, application Great Britain Mar. 3, 1959 6 Claims. (Cl. 201-) This invention relates to the production of extruded metal products and more particularly to a method for the high speed extrusion of products of varied cross sectional configuration and area, and improved extruded metal products. This application is a continuation-in-part of commonly owned copending applications Serial Number 212,728, filed February 26, 1951, now Patent No. 2,753, 994, dated July 10, 1956, Serial Number 385,559, filed October 12, 1953, now abandoned, Serial Number 419, 478, filed March 29, 1954, now abandoned, and Serial Number 506,063, filed May 4, 1955, now abandoned.

In the art of cold metal extrusion as heretofore practiced, it is known that strain hardening of a billet or a portion of the billet while yet within the billet results from the plastic flow of the billet which is undergoing the compression within the extrusion equipment and which brings about the extrusion. The effect of this strain hardening of even a portion of a billet required to complete the extrusion is to impose upon the press ram a load which progressively increases in magnitude during the extrusion operation. It is apparent that due to this progressively increasing load, the capacity of a press is easily exceeded if an attempt is made to produce a very long extruded product with considerable reduction in area. Accordingly, the length of articles heretofore producible by cold metal extrusion has been quite limited. Cold metal extrusion as used herein refers to the starting temperature of the billet and means that, at the beginning of an extrusion, the billet is at room temperature or, in any event, at a temperature low enough so that it is still capable of work hardening.

While strain hardening elfects have limited the length of products heretofore producible by cold metal extrusion, long products can be produced by other methods. One of these methods is to preheat the billet to a temperature at which it is no longer capable of strain hardening and then to hot extrude. Here the ram pressures can be kept at reasonable levels, but the products of hot extrusion are characterized by the absence of many of the desirable properties of cold formed metal.

Of course, it is possible somewhat to mitigate work hardening effects incident to starting with a cold billet by preheating the billet some but not enough to destroy its capacity to work harden. The effect of this sort of preheating is to extend somewhat the capacity of a press, but the Work hardening effect of the metal still within the billet is nevertheless felt long before the metal is extruded to a really great extent if the billet is compressed at a velocity such as those commonly used heretofore.

In the metal working arts it has been customary, in cases where a large extension ratio is sought and a Work hardened product is required, to accomplish the result by reducing wall thickness by an initial extrusion at small extension ratio, annealing, further reducing Wall thickness as by additional extrusion operations or drawing, further annealing and so forth with additional deformation steps until the desired thinness of Wall of the product is achieved. While by this procedure one can achieve very high over all extension ratios, the cost of the product is very high due to such things as the handling incident to the intermediate annealing and re-working steps.

This was the state of the commercial development of extrusion when the applicant entered the field.

It is an object of the present invention to overcome the deficiencies of the prior art and to provide a method for the single stroke impact extrusion of billets characterized by room starting temperatures or, in any event, temperatures low enough so that the metal is capable of strain hardening, avoidance of increase in ram load during the plastic flow and extrusion of the metal, and the production of articles of desirable properties and great length at any desired extension ratios, even over 100, extension ratio as used herein being the quotient of the length of the extruded product divided by the length of the billet to be extruded.

It is proposed, according to the present invention, to compress a billet which is at room temperature or, if heated, at a temperature at which the metal or metals constituting the billet is or are capable of strain hardening, at a strain rate or rate of deformation and at a stress or load value so generated and maintained as to produce plastic flow of the metal or metals without increase in the ram pressure requirements during the extrusion and yet to produce an article characterized by desirable attributes of cold-formed metal. In general, strain is used to define the unit deformation of a metal and stress is used to define the unit force which causes it.

Another desirable feature of the present invention is the applicability of the method hereof to metals which exhibit a marked tendency to strain harden when extruded by conventional methods whereby it is possible to extrude in extrusion times from a small fraction of a second upwardly, and in a single stroke from a room temperature billet of such a metal, an extrusion of great extension ratio.

A further object of the invention is to produce, from strain hardenable metal and exclusively by extrusion, an extrusion completed article characterized in that it is of large extension ratio, but nevertheless, at the time of its extrusion completion, is relatively highly and substantially uniformly strain hardened as compared to extrusion formed articles previously produced.

Other objects and advantages of this inventionwill be apparent from a consideration of a detailed description of the general method and of specific examples thereunder in conjunction with the annexed drawings wherein:

FIGURE 1 is a schematic view in vertical section of a typical extrusion press equipment arrangement capable of carrying out the method of the present invention;

FIGURE 2 is a true stress-true strain diagram depicting the behavior of a specimen stressed in tension at A;S.T.M. rates to the breaking point, as well as the energy which would be required to produce maximum elastic deformation of the specimen at deformation rates in accordance with this invention;

FIGURE 3 is a true stress-true strain curve depicting the yield point or origin of plastic flow in a specimen stressed in tension;

FIGURE 4 is a true stress-true strain diagram depicting the behavior of the same metal when stressed in tension at different temperatures and strain rates;

FIGURE 5 is a chart showing in graph form on the ordinate the critical velocities in compression of a number of metals and alloys arranged on the abscissa in ascending order of modulus of resilience;

FIGURE 6 is aview in elevation of a billet of one kind extruded according to the present invention;

FIGURE 7 is a view in elevation to the: same scale as FIGURE 6 of a tube extruded from the billet of FIG- URE 6;

FIGURE 8 is a true stress-true strain diagram of a specimen stressed in compression at substantially A.S.T.M. rates, also showing the maximum strain value (e of the specimen material when stressed in tension at such rates;

FIGURE 9 is a pressure-velocity curve showing the influence of velocity of compressing a billet during extrusion upon the pressure exerted upon the billet;

' FIGURES 10 and 11 are respectively photomacrographs of specimens of split partially extruded billets etched with aqua regia;

FIGURES 12, 13 and 14 are photomacrographs of split test billets provided With lead inserts and extruded respectively one-fourth, one-half and three-fourths of complete extrusion strokes, stained with sulphuric acid;

FIGURES 15, 16 and 17 are photomicrographs of small test billets of pure aluminum subjected to scratch hardness tests at varying time intervals following plastic deformation.

The A.S.T.M. rates referred to herein are well known, being disclosed, for example, in ASTM STANDARDS, 1952, published by the American Society for Testing Materials, Philadelphia, Pennsylvania, Part I, pages 1397- 1399 (Ferrous Metals), and Part II, pages 12161218 (Non-Ferrous Metals).

Referring briefly to the drawings, a form of extrusion equipment suitable for use in practicing the invention is shown as including a ram 10 formed as a container providing a chamber 13, and a die 11. The billet to be extruded is represented by numeral 12, and is accommodated during extrusion in the chamber 13 in the ram 10. A pilot 14 is disposed centrally within the chamber 13 so as to be projectable concentrically into the die aperture with clearance. After extrusion, the discard is left on the face of the die 11 for removal. A ram and die assembly of the type depicted in FIGURE 1 is shown in application Serial No. 212,728, filed February 26, 1951, now Patent No. 2,753,994. A press suitable to maintain high continuous ram speeds is shown in application Serial No. 295,097, filed June 23, 1952, now Patent No. 2,984,- 980. Both of the above-mentioned applications are owned in common with this case.

During operation, the billet is positioned adjacent to the die aperture and the ram 10 is suddenly and substantially instantaneously caused to subject the billet to pressure at a high strain rate to extrude the metal of the billet through the die orifice to produce an elongated product of the desired cross sectional configuration. The suddenness of pressing of the billet is such as to be in the nature of an impact. As used herein impact is intended to include sudden application of force not necessarily preceded by free travel of the ram.

In the early work of this applicant on aluminum and aluminum alloy billets he discovered that high ram pressures and the imposition and maintenance of high strain rates throughout the extrusion resulted in the production of extruded articles of unusual length, unusual uniformity and degree of work hardened condition throughout the lengths of the extruded sections, and unusual thinness in cross section without the increase in ram loading normally expected as a consequence of strain hardening. It has been found that if the ram speed at the point of impact of a billet to be extruded is above a critical velocity for the particular metal of the billet and for the particular extrusion operation to be performed, and that if the ram velocity during compression is maintained at or above the critical velocity for the full extrusion, stroke, billets of large volume may be extruded at high extension ratios, herein represented by the symbol A, by a single stroke of the ram of the press. For example, it is possible to extrude a 2 S aluminum billet of 4%" length at an extension ratio of about 60, resulting in a product more than 20 feet long.

In copending application Serial No. 212,728, now Patent No. 2,753,994, are described an apparatus and methodfor forming, by cold percussive extrusion, long lengths of thin walled extruded sections having a high degree of cold working. The method is characterized by the sudden or substantially instantaneous application of ram pressure to the billet to be extruded, and an uninterrupted follow-up ram stroke with the ram impacting the billet percussively and operating throughout the extrusion at a pressure and velocity so correlated as to maintain the percussive nature of the extrusion throughout the press stroke to produce a very long extruded section. The degrees of pressure and velocity and their correlation are evidenced and measured by the high degree of cold working effect and subsequent low elongation value in the long extrusion completed product. In this way, single stroke extrusion operations are performed at high extension or extrusion ratios to produce long, thin walled and highly cold worked extruded sections. Extrusion ratio is the ratio of the cross sectional area of the billet to the cross sectional area of the extruded section, and, ignor ing a small discard, is numerically the same as the extension ratio for any extrusion operation.

An example of the scope of cold percussion extrusion according to this invention, as disclosed in copending application Serial No. 212,728, now Patent No. 2,753,994, is illustrated in FIGURES 6 and 7. In those figures a substantially pure aluminum billet of 2 inches in length was extruded into a tube more than 8 feet in length and having a wall thickness of .020 inch, in this example the outside diameter being .415 and the inside diameter being .375". By using the method of the present invention it is possible to form such extrusions of great length with wall thicknesses of well under .040 inch. The ram moved at a velocity between 10 and 20 inches per second and the pressure on the ram was at about 75 British tons per square inch. As set forth herein, the single stroke extrusion of a tube more than 8 feet in length from a 2 inch long billet represents a A or extension ratio of about 51. Some idea of the order of magnitude of a A of 51 can be appreciated by reference to FIGURES 6 and 7 which are drawn to scale in relation to one another. Note that the outside diameter of the billet was 1 inches while the inside diameter of both the billet and the tube was .375 inch.

With the knowledge thus gained, applicant was able immediately and more easily to go forward with commercial production of cold extruded thin wall sections a number of feet in length. He found he was able by virtue of this knowledge supplemented by appropriate testing to successfully cold extrude other sections in the same and different metals and accumulated considerable data as to the pressures and velocities applying to the various metals and sections and, of course, the extension and extrusion ratios.

Inasmuch as the amount of work and time necessary to determine these values for the different metals, sections and extension and extrusion ratios was considerable and consumed time, it was conceived that if close approximations of the pressure and velocities required could be had by the use of more or less generally applicable formulae or mathematical calculations, the time required for predetermination of pressures and velocities for each new job might be very considerably foreshortened. Utilizing the accumulated data of experiment and production, more or less generally applicable formulae were arrived at and will be set forth below. While they are to some extent at least and perhaps to a considerable extent in one case or another empirical in nature, they are, nevertheless, sufficiently accurate to save much time in very approximately predetermining pressures and velocities to conform to the invented process.

By reason of the fact that successful cold extrusions can be made over a considerable range of velocities above the critical velocities, albeit the physical properties of the resultant extrusion may not remain the same throughout the range, conditions requiring adjustment from the calculated velocities can easily be met.

In evolving any formula applicable to the extrusion of different metals or alloys it is of course necessary to know something about the physical properties of each particular metal or alloy and the extension ratio desired. The necessary properties of the metal may be derived from an ordinary true stress-true strain curve such as is available for most metals. Such curves are prepared by stressing a specimen in tension at an A.S.T.M. strain rate until it ruptures, and plotting increments of stress required to produce increments of strain, taking into account the change in cross section of the specimen resulting from the strain. In cases where there is no available true stress-true strain curve, as when new alloys are developed, one may easily produce one by well known procedures.

Prior to the filing of application Serial No. 385,559, now abandoned, the applicant had come to the realization that there was a critical minimum velocity or strain rate at and above which the metal of the billet ceases to exhibit the heretofore encountered strain hardening properties on a billet or a portion of a billet required to complete the extrusion while still within the extrusion equipment. This has now been confirmed. In application Serial No. 385,559, now abandoned, the rate at which the metal is deformed is described as strain rate in inches per inch per second. On a theoretical basis which disregards the actual change in size of the metal billet, the term is entirely correct. However, because it is somewhat difficult to measure the values from which strain rate is calculated, the specification in copending application Serial No. 419,478, now abandoned, and the present specification refer not only to the rate of deformation per unit length of the metal, but to the velocity of the ram which deforms it or, in other words, the velocity of compression, it being understood that the rate of deformation of the metal is the critical thing Whether it be described in terms of strain rate or ram velocity or velocity of compression of the billet as a whole.

In supplement to the extended data available from the applicants early experiments and production jobs, there were made from a number of different metals and alloys an orderly series of extrusions at different extension ratios accompanied by tests for physical characteristics. The nominal chemical composition of some of the metals and alloys in this series of tests is given in the 5th Edition of Specifications for Aluminium and Aluminium Alloy Products published by Northern Aluminium Company Limit- A=extension ratio or length of extrusion divided by length of billet to be extruded.

In the above formula, it is apparent that the value of A is related to the article being produced and that the value V is related to the particular metal being used. It has been discovered that there is a correlation between the ram velocity V and the capacity of a metal to absorb maximum energy by elastic deformation. One way, in which V which is a property of the metal and is set forth in the foregoing formula, may be determined is illustrated by FIGURE 2 and the following formula:

a =the true stress in p.s.i. at the rupture point of the metal under tension.

e the true strain under tension in inches per inch at the end of the extrapolated elastic deformation line on a true stress-true strain curve.

Density (pounds/in. (acceleration of gravity in inches per sec. per sec.)

An index of the modulus of resilience may be used as a property of a metal in readily determining ram velocities for extruding that metal. Both the early and later experimental tests and production work indicated that at ram velocities of V or greater there is no increase in load during the extrusion. This suggests a flat curve 0 to the right of the vertical line W-e (FIG. 2) representing stress in the plastic region. The slope of the curve in the elastic region, B, is of course, the modulus of elasticity and this must intersect the plastic flow curve. The area e We under the line of slope E from s to e is a measure of the potential energy per unit volume which may be absorbed elastically and is called the modulus of resilience, which may be expressed in inch-pounds per cubic inch. This measure of potential energy may be equated to a corresponding measure of kinetic energy of which velocity is a component. The above formula is one Way of extracting a velocity component which is an index of velocity required to achieve a kinetic energy equal to or greater than the potential elastic energy expressed =mass density or 384 ed of London, England, as follows: 5 in the shaded triangle of FIGURE 2.

[All figures are percentages] 011 Mg Si Fe Mn Ni Zn Ti Cr 2 S 0.1 0. 5 0.7 0.1 IMP 0.05 51 S 0.15 0. kl. 5 0 75-1. 3 0. 6 1. 0 IMP 0.1 0 2 O. 5 26 S 3. 5-4. 8 0. G 1. 5 1.0 1. 2 IMP 0.25 IMP 0.2 0 3 77 S... I--. 3. O *4. O 0. 6 0. 6 1. 0 *4.0*8.5 0 3 1. 0

*When the zinc content is in excess of 8.0%, the magnesium content shall not exceed 2.5%.

V =the minimum ram speed at impact and thereafter during the stroke in inches per second required to pro duce an extruded section Without increasing the ram loading during the ram stroke.

V =index of velocity of deformation under tension required to achieve a kinetic energy equal to or greater than the elastic energy per unit volume which can be absorbed by the particular metal prior to ideal plastic deformation.

The curve of FIGURE 2 is an ordinary true stress-true strain curve of a specimen stressed in tension at an A.S.T.M. testing strain rate. Of course, the slope of the line h which is an indication of strain hardening, influences the value of stress at rupture and hence the stress value of line c as do the modulus of elasticity E and the yield point. If the billet is somewhat heated before extrusion, the slope of the line It is reduced as are the modulus of elasticity and the yield point, and the stress 0 is therefore reduced with consequent reduction of V and V When a billet is to be somewhat preheated before being extruded in accordance with the present invention the stress-strain curve indicated properties are properly determined on the basis of a specimen tested at approximately the temperature to which the billet is to be preheated.

In the operation of a press for the cold extrusion, in a single press stroke, of all of the metal of a billet required for a long extrusion at high extrusion ratio, except for a small necessary discard, the billet is compressed at or above the velocity V and is continued to be compressed at or above that velocity for the entire stroke of the machine.

Naturally a working pressure sufficient to generate and maintain the calculated velocity of compression V must be present. In calculating this pressure, one may consider the ductility of the metal, the instantaneous yield point of the metal and the extension ratio. These can be calculated in units so that they are applicable to a billet of any size. It has been found with reference to this invention that the instantaneous yield point of the metal at the velocity required for the practice of the invention is higher than the yield point when the material is strained at an A.S.T.M. static test rate. Once the material has exceeded its yield point and is plastically flowing through the orifice, the instantaneous yield point will be affected by the ductility of the metal as well as the size of the opening through which it is being extruded. The strain in the metal (e at the instant of rupture in the tensile test is an index of ductility, see FIG. 3. A, which represents the extension ratio, depends on the size of the die opening and the cross section of the billet.

If one expresses these relationships mathematically, the following formula results:

P=YP+2YPAe in pounds/sq. in. of billet area.

YP=yield point in p.s.i. under static test i.e. load per unit area applied at A.S.T.M. test rates, or strain rates, in tension.

A=extension ratio as previously defined.

e =strain magnitude in inches/inch of deformation at rupture, in tensile test.

P pressure on billet in pounds/sq. in of billet area.

It will be understood that the pressure above applies only for velocities equal to or above the critical velocities of the ram V as defined herein.

Now, having selected a particular metal and having operated the press for the correct values of V and P as given in the foregoing formulae for starting with a billet at room temperature, the stress-strain pattern for the extrusion follows substantially the contours of line 16 of FIGURE 4. It can be noted that the extrusion, once begun at a velocity V in accordance with the present invention, does not require higher and higher stresses to maintain the continuing strain as is the case where the strain rate is below V see line 15.

The curve 18 marked Hot in FIGURE 4 is based upon a preheated test specimen of the same alloy or metal as that portrayed in the line 16 marked Cold. Note that the effect of preheating is to reduce the stress required for a given strain. The resulting value of V is smaller and V is smaller. If, however, the rate of compression is below V for the selected starting temperature, the stress required to produce the desired strain will increase during the extrusion and produce a curve such as 17 in FIGURE 4.

It should be explained that due to the flatness of the plastic curve in a true stress-true strain diagram, it is possible to get a more accurate value for YP than had been heretofore contemplated. In FIGURE 4, which is taken from Serial No. 385,559, now abandoned, the X marks a yield point which may be described as the point on the curve where plastic deformation begins. For the sake of accuracy, YP in the formulae herein is measured as the intercept of an extension of the straight line portion of the plastic curve of a true stress-true strain diagram with the stress axis, see FIGURE 3. When it is said herein that the yield point at velocities of compression at or greater than V is greater than the yield point of the same specimen under static test, it is realized that small error may exist in measuring the low yield point value due to the fact that mixed plastic and elastic deformation are occurring and it is difficult to locate a yield point with high accuracy. The directions here given for operating the press to get the benefits of the present invention are so prepared as to give a margin of safety in cases where small errors of measurement are made.

Some conception of the time saving achieved by the use of such formulae may be obtained by reference to the following table of minimum ram pressures and velocities which were calculated from the formulae explained above.

Table I P, 0211- Calculated em, A (in./ culated Metal p.s.i. in./in. in.) p s.i. for

tubes V1, 111.] Va, 111.]

sec. sec.

3. 12 176,200 3. 6 6. 25 182, 000 3. 8 77 SO 24,000 .140 12. 5 188,000 842 4. 05 25. 0 196, 000 4. 27. 3 196, 400 4. 98 3. 12 86,120 1. 78 538 288 a; 1 .5 51 SO 10,500 .406 250 129,500 581 241 50. 0 155, 200 3. 14 100. 0 191, 900 4. 3.12 160, 000 2. 31 6. 25 167, 000 2. 42 26 S'O 21,500 .179 12.5 174,000 665 2. 63 25. 0 184, 000 3. 06 27. 3 184, 000 3. 12 12. 5 80, 200 0. 81 25.0 92, 800 1. 05 2 SO 7, 400 .423 50. 0 114,400 351 1.49 52. 8 116, 300 1. 54 100.0 140, 800 2. 37

In the above tabulations, 5 refers to the type of aluminum which is used for making wrought products, and O is a temper designation and refers to fully annealed metal (this terminology is taken from Alcoa Aluminum and Its Alloys, Aluminum Company of America, 1950). YP is the yield point in the formula set forth previously herein. emax, is as indicated in FIGURE 3. A is extension ratio as discussed and defined previously. The calculated P is the pressure resulting from use of the formula P=5YR+2YPAe V is the index of velocity calculated as described supra. Calculated V is determined from V by the use of the formula set forth supra.

In the Table I, supra, the values of P and V listed conform generally to the observed pressures and velocities in the actual production of extrusions. In actual operations, there may be a 10 to difference between measured pressure and calculated pressure. This should be expected since the formula for calculated P is for a theoretical minimum. Variations in the properties of different billets of the same metal or alloy, variations in the lubrication of the billet and a certain amount of unavoidable error in measurement make a safety factor necessary. The safety factor for the calculated P is about 10 to 15% and it has been found to be adequate to deal with billets within the expected range of deviation from normal for any given alloy.

In the actual extrusion runs, ram approach velocities were observed, and while the approach velocities, i.e. the ram velocities prior to the full impact on the billets, may differ appreciably and sometimes markedly from the velocities of billet deformation, the approach velocity and pressure can readily be adjusted to cause the velocity of deformation to meet the requirement of extrusion at or above the critical velocity according to this invention.

V is a minimum value below which substantial strain hardening effects on the billet within the extrusion equipment can be expected, the magnitude of which will vary with the metal being worked. Due to the fact that the V figures are for a minimum, shop practice will be to operate somewhat above V but not so much above V as to cause overheating the extrusion due to die friction. Products having optimum physical properties are produced at velocities a little bit above V Since V is calculated for the statistical average for the particular alloy, it would be expected that some billets or some parts of the billet would have a crystal arrangement such that the calculated V would not be suificient. The slight increase over V makes more certain that the critical velocity is exceeded for all parts of the billet with the result that a somewhat better appearing extrusion results. At values excessively above V heating effects from the die begin to bring about localized thin areas, burning and tearing. Experience indicates that this deterioration has nothing to do with the metallurgical theory of the present invention, but is due mainly to problems of die lubrication long known to the art. For some particular dies these problems may become troublesome only at very high actual velocities. For some extrusion ratios and some sections of some metals, the critical velocities V may even be multiplied several times and a complete and usable extrusion formed, but it nevertheless is preferable to employ velocities exceeding V more moderately when it is desired to retain the advantageous effects of cold working and thus to enhance the physical properties of the extruded product. In some cases where different properties are desired deformation velocities considerably exceeding the minimum V may advantageously be employed. The pressure formula P YP-I2YPAE includes an appropriate safety factor for the extrusion of annular sections such as tubes.

Since the metal is flowing by plastic deformation when it is compressed at velocities at or above V the Bernoulli phenomena are encountered which means that even with the same A and the same die area, ram loadings may have to be changed in accordance with the shape of the section which defines the area. This will result in an upward or downward adjustment of P. It is noted that all of the figures given in Table I, supra, are for annular sections, although re-entrant sections, rods and many other shapes have been successfully extruded.

It is important to remember that a velocity of at least V dictated by the particular metal and its extension ratio (A) must be maintained throughout the deformation operation. With a change in shape in cross section of the product to be formed the pressure required to maintain the velocity at at least the dictated V may need to be adjusted.

Another mathematical correlation of test and production data led to the evolving of another set of formulae explained in copending application Serial No. 506,063, now abandoned and below, and which, in many cases, provides for more accurate predetermination of the requirements of pressure and velocity of deformation required for the successful practicing of this invention over a greater range of extrusion operations involving different metals, shapes of extruded sections, and extrusion and extension ratios. For these reasons, the set of formulae explained below has come to be preferred in the predetermination of minimum pressures and velocities of deformation.

The most nearly accurate predetermination of the minimum pressure required for maintaining the minimum ram velocity V is of course to be desired for practical and economic reasons, particularly for long die life. The pressure exerted on the slug and the consequent pressure exerted by the slug on the die during extrusion have a marked effect on die life. Dilferent die opening shapes or cross sectional configurations of extruded sections, other factors being equal or substantially so, introduce different pressure requirements, and the copending application Serial No. 506,063, now abandoned, previously referred to discloses a method for determining different required or minimum pressures according to different die opening shapes or extruded sections, e.g. for regular solid sections, for regular tubular sections, for irregular solid sections, and for irregular hollow sections. The relationship of unit pressure required by the present invention to be exerted on the slug or billet and factors which are known or ascertainable according to the properties of the metal to be extruded and the nature of the extrusion operation is expressed as follows:

- b max.) in which:

P is the loading pressure per unit area of die face for forward extrusion or per unit area of the ram cross section for backward extrusion, in British tons (2240 pounds) per square inch, exerted on the slug or billet,

a is the stress per unit area, in British tons per square inch, on the true stress-true strain diagram of the metal in compression shown in FIGURE 8 at the maximum strain value (e obtained in tension (see FIG. 3),

e is the natural or maximum strain, e.g. in inches per inch, at the ultimate strength or point of fracture in the tensile test,

6 is the extrusion ratio, i.e. the ratio of the area of the slug to the area of the extruded section, which, ignoring a small discard or unextruded part of the billet, is numerically the same as the extension ratio (A), i.e. the ratio of the length of the extruded section to the length of the extruded slug or billet, and

K is a numerical constant which for regular solid sections is 1:9; for regular tubular sections is 2.1; for irregular solid sections is 2.3; and for irregular hollow sections is 2.5.

In copending US. application Serial No. 506,063, now abandoned, the pressure P in the formula:

Pressure=Ka l emax. 5

is represented by the letter Y instead of by the letter P and the extrusion ratio is represented by the symbol A instead of 6, as above.

Determination of the pressure P required for effecting extrusion in accordance with the present invention on the basis of the relationship P =Ka(1-i-'e )6- not only enables the fixing of pressure more suitably according to the shape of the section to be extruded, but provides a further procedure for determining the ram velocity during the extrusion operation best suited to the operation to be performed. It has been determined that the pressure p at constant ram velocity V is proportional to a function of the extrusion ratio 5, and that at constant 8 the pressure is inversely proportional to a function of the velocity V. Thus:

poo f6 (at constant velocity) and p w J% (at constant 8) In accordance with this invention, the relation of pressures P the minimum or critical ram velocities V in inches per second required to be maintained throughout extrusion operations on metals capable of plastic deformation and preferably in the annealed state, the metal properties, and the extrusion or extension ratios has been determined to be:

in which,

P is the loading pressure per unit area, e.g. in British tons per square inch, exerted on the billet or slug,

TI is the yield point in tension or compression of the billet metal, in British tons per square inch, as determined by the intersection of the extension of the straight line portion of the plastic deformation curve with the stress axis, as shown in FIGURE 8 (in application Serial No. 506,063, now abandoned, this intersection is denoted by M and is termed the modulus of resilience),

e is 2.718 (base of Napierian logarithms),

is the extrusion ratio,

C 5 a being the true stress in compression in British tons per square inch at maximum strain (e of a tension specimen, and

V is the minimum or critical ram velocity in inches per second required to be maintained throughout the extrusion operation. One example (A) of performing an extrusion operation in accordance with this invention and as determined by these relations of factors is the extrusion of a tube from an aluminum slug (99.6% to 99.8% pure aluminum),

the slug having an outer diameter of 1.75 inches, an inner diameter or bore of .718 inch, and a height or length of 2.75 inches, the tube to be produced with an outer diameter of .718 inch at a reduction in area of 98%, that is to say an extrusion ratio of 50. The properties of the metal are True yield stress in tension:1.45 tons per square inch,

Stress a in compression as defined above=7.5 tons per square inch,

E 422 inch per inch intension, and 5 W (yield point as explained above, in British tons per square inch intension or compression) :4.

Since the extruded section is to be a regular tubular section, the constant K in the relation b max.) is 2.1. The pressure P in tons per square inch required to be exerted on the slug is established to be 59.5, a total ram loading of 119 British tons being required since the area of the slug is 2 square inches. For this same example, the required minimum or critical velocity is determined by the relation to be 3.64 inches per second.

It is advisable, in order to insure completion of the ex trusion, to provide a press having a capacity of at least 10% to 15% greater than the maximum pressure value determined by the formula discussed above. It is also advisable that the minimum or critical ram velocity to be maintained during the extrusion operation as determined above be increased somewhat in practice, but not by more 12 than up to 2025 and seldom by that much. Too large increases over the minimum or critical velocity V may produce heating effects from the die, resulting in defective extrusions due in part to lubrication difficulties.

As another example, (B), a magnesium silicon aluminum alloy known as H.9 British Standard is extruded from a slug having the same dimensions as those of the slug in example (A). The chemical analysis of this alloy as set forth in the 5th edition of Specifications for Aluminium and Aluminium Alloys published by Northern Aluminium Company Limited, London, England, is:

Cu (Imp) 0.15 Mg 0.40-090 S1 0.300.70

all figures being percentages.

The extruded section is of the same dimensions as in example (A), the extrusion ratio 6 again being 50. In this case the yield point YT as explained above, is 5, the stress a is 8.8, and the strain a is .409. The relation P =Ka(1 +e )6- determines the unit pressure to be exerted on the slug as 69.4 tons per square inch and the total ram loading as 138.8 tons.

The relation 5 b YF CVR determines the minimum or critical ram velocity V to be maintained during the extrusion operation to be 4.11 inches per second.

Further examples of minimum pressures P in tons (2240 pounds) per square inch and critical ram speeds V in inches per second determined from the relations P =we CVR respectively, for extrusion of regular tubular sections from billets 1% inches long, 1% inches outside diameter and inch inside diameter are set forth in the following table:

Table II Pressure in Pressure in Velocity in Velocity of British tons/ British tons/ i11./see. from deformation 6 in. from in. recom- Formula in in./see. Metal (inJ/infl) Formula Pb mended for recommended a( mux.) production for produc- 6 operations Pb=YPG CVR tion operations Cu OFHG The compositions of the metals or alloys listed above in Table II are set forth previously in this specification.

To give some idea of the range and scope of the invention and what happens when the ram speeds are not maintained at or above V or are excessively above V five billets of 77 S-O aluminum each 1 /8" high and having an outside diameter of 1 3 and an inside diameter of A were extruded under identical conditions except for variations in the velocity of the ram and consequent pressure changes. All of the examples were cast from the same heat or melt of the 77 S-O aluminum. They were all at room temperature and at the same temperature at the be ginning of the extrusion operation. The first billet was compressed at a ram speed of /4 a second, and the press stalled after extruding only of tubing. The pressure on the billet went up to 326,000 pounds per square inch which was the capacity of the particular press used. The second billet was sought to be extruded at a billet compressing ram speed of l per second. This extrusion was also unsuccessful due to breakage of the pilot. The third billet was extruded at 6" per second billet compressing ram speed at a pressure of 233,000 pounds per square inch of billet area. The extension ratio A was 25 and the extruded product was good. A fourth billet was extruded at a billet compressing ram speed of between 6" per second and 13 /5 per second, the precise velocity not having been determined. In this case the extension ratio A again was 25, but the extruded product was scored and flaked. A fifth specimen, which was extruded at 15" per second billet compressing ram speed, appeared to have thin spots and transverse score marks, apparently due to the heat effects at that speed. The calculated V for 77 8-0 aluminum at an extension ratio of is given in Table I, supra, as 4.60 inches per second; and the calculated value V of the same metal at an extrusion ratio of 25 is given in Table II, supra, as 4.95 inches per second. A production operation velocity of deformation of 5.00 inches per second is recommended in Table II for this extrusion operation. Thus, the results of the series of tests of extruding 77 8-0 aluminum at progressively increasing velocities of deformation confirm the usefulness of the formulae in predetermining the critical velocity of deformation within approximately 25 percent. While the extrusion of the fourth and fifth 77 S-O specimens at ram speeds considerably above the critical speed produced completed sections at an extension ratio A of 25 which might be used for non-critical purposes, extruding at a somewhat lower ram velocity, say 5 inches per second. or 6 inches per second, that is at or a little above the critical velocity V is preferable for producing extruded sections having superior physical characteristics such as hardness and strength, and superior surfaces.

Critical or minimum ram velocities and pressures required to be maintained throughout extrusion operations in order to produce extruded sections in accordance with the present invention have been determined by the actual production of many extruded sections of different metals, different shapes and with different extrusion or extrusion ratios. Experimental or testing and extensive manufacturing or production experiences have shown that when deforming a billet at or a little above a critical ram velocity it is possible to produce an extrusion-complete product of metal which is so capable of being strain hardened that a portion of the billet required to complete the extrusion would become strain hardened while yet within the billet to such an extent as to cause a major increase in the pressure, or even to stall the press, it deformed at a relatively low velocity less than the critical velocity. The critical velocity is above the highest of the relatively low velocities at which strain hardening sufiicient to stall the press or cause a material rise in pressure can occur in any part of the billet required to complete the extrusion while still within the billet. Consequently, the metal of the billet, except for any discard intentionally left within the container after the extrusion operation has been completed, will all be forced through the die and formed into an extrusion-complete article before strain hardening within the billet can stall the press or even cause a substantial rise in :the extrusion pressure and ram loading. The total time required for performing the extrusion operation of sections many feet in length is always in excess of the time within which strain hardening of a portion of the billet required to complete the extrusion would take place in that billet portion while still within the billet if deformed at a velocity below the critical velocity. This time factor is taken into consideration in the method according to this invention.

The importance of maintaining the rain speed at or above a minimum V as explained herein may be appreciated further by consideration of FIGURE 9 showing the influence of velocity of compression on the pressure exerted on billets during extrusion. It will be seen that at relatively low velocities relatively high pressures are exerted, whereas at relatively high velocities, relatively low pressures are exerted. At velocities at and above the minimum velocity V according to this invention, the pressure does not increase within the usual operating ranges of velocities imposed by desired physical characteristics of the product, as shown by the parallelism of the Pressure-Velocity curve to the right of V to the velocity (x) axis in FIGURE 9. The shape of the curve to the left of V clearly shows the increase in pressure at velocities below V In some cases, the pressure curve over the extrusion stroke indicates that the resistance of the billet lessens toward the end of the stroke. The curve shown in FIGURE 9 was plotted from observed pressure and velocity values in extruding billets of 77-8 aluminum all at a A or 6 of 6.25 and at different velocities and pressures.

The relation expressions or formulae set forth in this specification are based on critical or minimum pressures and velocities which have been determined by tests and production experience to be the pressures and velocities required to be used for performing single stroke extrusion operations according to the metal properties, the kinds or shapes of the extrusion-complete sec-tions to be formed and the extrusion or extension ratios. The determinations of minimum pressures and velocities can be made with somewhat greater accuracy when the billets, and the corresponding specimens, are of homogeneous, annealed wrought metal than when of cast metal, this being due to various casting defects in the latter which, not being worked out, introduce discontinuities in the test piece specimens in tension and in the billets. Nevertheless, single stroke, extrusion-complete sections can be extruded from east billets in accordance with this invention and on the basis of the critical pressure and velocity determinations explained above.

An essential requirement for successful practicing of the present invention in producing a single stroke. extrusion at a high extension or extrusion ratio from a billet at a temperature at which it may be strain hardened is that the billet be compressed at a velocity high enough at all times during the extrusion operation to cause the billet portion being subjected to plastic working or deformation to be expelled from the billet and the die equipment faster than strain hardening can set. in while the plastically worked metal is still within the billet. It is believed that, when a billet is compressed at or above a critical ram velocity V in accordance with this invention, it is mainly the billet metal at any time adjacent to the die aperture which undergoes plastic deformation and which is therefore conditioned for strain hardening, the metal more remote from the die aperture and closer to the ram undergoing little if any plastic deformation; This is illustrated graphically in FIGURES 10-14 inclusive, each of which shows a split partially extruded 2S aluminum billet. FIGURES l0 and 11 show partially extruded billets and extruded tubular portions etched with aqua regia, the billet shown in FIGURE 10 having been extruded at an extrusion ratio A of 25 and that shown in FIGURE 11 having been extruded at an extrusion ratio A of 10. The extrusion strokes were stopped suddenly when partially completed. In each of FIGURES and 11 the darker area at the extreme bottom portion of the billet, i.e. that portion which was immediately adjacent to the die aperture, may be recognized at once as showing plastically deformed metal. The more mottled or lighter area extending upwardly from the region of plastic deformation illustrates billet metal which, at the time of stopping the extrusion stroke, had been mainly or substantially only elastically deformed and had not begun to flow plastically to any substantial extent. Each of FIGURES 12, 13 and 14 shows a split billet which, prior to partial extrusion, had been drilled and loaded with lead plugs extending both longitudinally and chordally. The surfaces of split were stained with sulphuric acid, and the black or very dark areas are the lead plugs or inserts. FIG- URE 12 shows the condition of a billet after having been extruded only one-fourth of a complete extrusion stroke, the in-turned lower end of the longitudinally extending lead plug depicting the localized plastic deformation and and the direction of plastic flow toward the die aperture. No one of the three chordal plugs shown in FIGURE 12 having descended to the active plastic deformation zone (see FIGURES 10 and 11), the chordal plugs have remained substantially in their original condition save only for a slight flattening due to initial consolidation of the billet incident to its lateral expansion within the container and possibly also due in part to physical weakening of the billet structure by the substitution of relatively weak lead for the stronger aluminum removed from the chordal drillings. FIGURE 13 shows a similar billet after having been extruded approximately one-half of a complete extrusion stroke, the localized flow in the restricted zone at the bottom of the billet again being indicated by the delineation of the longitudinally extending lead plug. Although this billet has been half extruded and one of the three chordal plugs has passed on through the die aperture, the next succeeding chordal plug, that is the lower of the two chordal plugs shown in FIGURE 13, is still spaced sufiiciently above the die aperture not to have entered the active zone. Consequently, this chordal plug, too, substantially has retained its original shape. FIG- URE 14 shows a portion of a split billet which has been extruded three-fourths of a complete extrusion stroke. Again, the longitudinally extending lead plug indicates the metal flow in the restricted active plastic deformation zone. The single remaining chordal plug, having moved into the upper part of the restricted active flow zone, has been deformed into substantially teardrop shape by the plastic working of the surrounding aluminum billet metal.

In consequence of the restriction of substantial plastic deformation to an active zone immediately adjacent to the die aperture, it follows that if each successive portion of the billet brought adjacent to the die aperture and there plastically deformed is forced through the die within a time less than the time delay between plastic deformation and the resultant strain hardening the extrusion operation may continue without substantial increase in pressure.

FIGURES 15, 16 and 17 are photomicrographs of pure aluminum test specimens graphically illustrating the time delay for strain hardening following extrusion. These figures respectively show small pure aluminum test specimens, all of the same metal, which have been subjected to scratch hardness tests at different time intervals following extrusion. The specimen shown in FIGURE has two scratches S1 and S2 made by identical scratching needles under equal loading, but at different intervals after extrusion, the scratch S1 having been formed at a time interval T1 of .28 second following plastic deformation by extrusion, and the scratch S2 having been formed at a time interval T2 of .34 second following extrusion plastic deformation. As is known in connection with standard scratch hardness test procedures, the width of a test scratch is an index of hardness. Since the scratches S1 and S2 in FIGURE 15 are wide rather than narrow and of substantially equal widths, it is evident that no hardening of the specimen took place between the making of the two scratches, indicating that as long as .34 second after plastic deformation strain hardening had not set in. The specimen shown in FIGURE 16 has two scratches S1 and S2 made by identical needles and under the same equal load ing as the specimen shown in FIGURE 15, but respectively at time intervals T1 of .36 second and T2 of .42 second after extrusion. The substantially greater width of the scratch S1 than the scratch S2 evidences hardening of the specimen at a time between .36 second and .42 second following extrusion. On the assumption that strain hardening set in approximately midway between .36 second and .42 second following plastic deformation, the indicated time delay for strain hardening would be .39 second. The specimen shown in FIGURE 17, also scratched by identical needles and under the same equal loading as the specimen shown in FIGURE 15, has two scratches S1 and S2 formed respectively at time intervals T1 of .68 second and T2 of .76 second following extrusion. Since, while the scratches S1 and S2 in FIGURE 17 are not noticeably ditferent in width and since both are more narrow than the scratches S1 and S2 in FIGURE 15 but not more narrow than the scratch S2 in FIGURE 16, it is apparent that no measurable increase in hardness took place between .42 second following extrusion and .76 second following extrusion.

When extruding long billets, the total time required for the ram to perform the extrusion stroke at a velocity in accordance with this invention is greater than the time delay between plastic deformation and strain hardening of the billet portion at any time adjacent to the die aperture. Nevertheless, if the ram velocity is maintained at or above the velocity required to expel any such so positioned billet portion from the billet in a time less than the delay time for strain hardening within the body of the billet, the extrusion can be completed in a single stroke, without intermediate annealing. The strain hardening will take place in the extruded section after it has been expelled from the die equipment, but this is extremely advantageous in producing extruded sections with such superior physical properties as in most cases to eliminate the necessity of subjecting the section to heat treatment or other finishing procedures.

It should be appreciated that, when, in accordance with this invention, ascertaining the minimum ram loading pressure per square inch of die face necessary for extruding some metals, the said minimum pressure may require the use of such high velocities of the extrusion ram that are not practicable, and/or such high loading of the ram that no known die steels or alloys can withstand. In such cases, in order to be able successfully to extrude such metals, it may be necessary to preheat the slug in order to render the physical properties of that slug such that they are comparable with those of a metal slug which can be extruded without preheating, and the term cold extrusion as used herein is intended to include such relatively slight preheating of the slug. In the practicing of this invention, a billet should not be pre-.

heated above the highest temperature at which the billet metal would be capable of work hardening sufficiently to stall the press before completion of a single stroke extrusion if the velocity of deformation were below the critical velocity V produced from moderately preheated billets according to the present invention, the metal properties used as control.

When extruded sections are to be.

extrusion may take place in that portion while still within the billet; but the ram speed should be below the velocity at which undesirable effects such as transverse tears, striations or markings are developed in the completed product. While these effects provide a criterion for determining the maximum velocity at which a billet may be compressed and a usable section produced, employment of velocities not exceeding the critical velocity V by more than 20-25 percent assures obtaining the higher physical properties. These are highly desirable in many products, especially thin walled aluminum sections. Where lower strength and greater ductility are required, the extrusion velocity may be markedly increased, for as heretofore indicated, successful cold extrusions can be produced over a considerable range of V values before defects occur in the products.

Although FIG. 1 shows die equipment for use in performing indirect forward extrusion, the invention may be practiced in performing direct extrusion operations, backward extrusion operations, and combined forward and backward operations. Press equipment of the kind shown in copending application Serial No. 295,097, now Patent No. 2,984,980, previously referred to is preferred for use in practicing the present invention, but other equipment capable of generating and maintaining the required ram pressures and velocities may be used. It is important, especially in the production of extruded sections of considerable length, for the press ram to be adapted or controlled to move at substantially uniform velocity and to exert substantially uniform pressure throughout the ex trusion operation in order that the physical properties of the extruded section be substantially uniform throughout the length of the section. Material variations in ram speeds and pressures while the section is being extruded may cause different portions along the completed section to have physical properties differing the extents which render such sections unsuited for uses where non-uniformity of properties is objectionable. By maintaining the ram speed substantially constant and within the range between the critical minimum speed V and the speed at which the objectionable conditions due to excessive heating set in, it is possible to produce extrusion formed and completed articles in the strain hardened condition with wall thickness of less than .040 inch and of unusual length, the metal of the extruded section having at the time of its extrusion completion a preferred crystal orientation, higher than normal yield strength, and higher than normal hardness.

I claim:

1. In the production by a single extrusion stroke of an extrusion complete article from a billet of a given strain hardenable metal under temperatures which continue the metal in strain hardenable condition throughout the stroke, which billet, if plastically worked within the die cavity under the extrusion ratio necessary to complete the extrusion of the article in the single stroke but under a velocity substantially less than the critical minimum velocity applicable to the metal when so extruded for a time greater than the delay time before setting in of strain hardening applicable thereto, will become consequentially strain hardened within the cavity, said critical minimum velocity being the lowest velocity which when maintained throughout a single extrusion stroke in the extrusion of a billet of a particular metal in strain hardenable condition at a predetermined extrusion ratio, will result in continuously expressing the successive plastically deformed portions of the billet from the extrusion die without causing a rise in resistance to forcing the metal out of the die cavity when the total time for performing the single stroke extrusion operation is greater than the time delay before setting in of strain hardening of each such successive portion of the billet brought adjacent the die aperture; the method which consists in initiating the single stroke and continuing it to the completion of the extrusion under a substantially uniform velocity at least substantially as high as and so related to the said critical minimum velocity that the resulting article is highly strain hardened and endowed with substantially uniform physical properties irrespective of the length of the article and the total time required for the extrusion.

2. The method according to claim 1 in which the velocity is not less than and is substantially equal to the said critical minimum velocity and the pressure is substantially uniform, whereby the energy required is reduced and the strain hardening of the article is substantially the maximum obtainable by such extrusion.

3. The method claimed in claim 1, wherein the critical minimum velocity conforms to that determined by the formula: V =V [V (5 10- +A(5 10 in which V is the critical minimu velocity in inches per second at impact and thereafter during the stroke required to produce an extruded section without substantially increasing the pressure on the billet during the stroke; V is the index of velocity of deformation under tension re quired to achieve a kinetic energy equal to or greater than the elastic energy per unit volume which can be absorbed by the particular metal prior to ideal plastic deformation; and A is the extension ratio or length of extrusion divided by length of billet to be extruded; and wherein the pressure P on the billet in pounds per square inch of billet area conforms to that determined by the formula:

Y]? is the yield point in p.s.i. under static test i.e. load per unit area applied at A.S.T.M. test rates, or strain rates, in tension; A is the extension ratio as previously defined; and e is the strain magnitude in inches per inch of deformation at rupture, in tensile test.

4. In the production by the process of extrusion in a single percussively instituted and percussively maintained impact stroke of an extrusion complete product from a billet of a given strain hardenable metal, under temperatures which continue the metal in strain hardenable condition throughout the stroke, which product requires so great a volume of billet metal subjected to the extrusion under the extrusion ratio necessary to complete the extrusion of the product in the single stroke that before the extrusion is completed there dwells within the diecavity for a time greater than the delay time before setting in of strain hardening applying to the metal a volume of metal needful to complete the product which if worked upon at velocities substantially less than the critical minimum velocity applicable to the metal when so extruded consequentially strain hardens 'while it is yet within the die cavity, said critical minimum velocity being the lowest velocity which when maintained throughout a single extrusion stroke in the extrusion of a billet of a particular metal in strain hardenable condition at a predetermined extrusion ratio, will result in continuously expressing the successive plastically deformed portions of the billet from the extrusion die Without causing a rise in resistance to forcing the metal out of the die cavity when the total time for performing the single stroke extrusion operation is greater than the time delay before setting in of strain hardening of each such successive portion of the billet brought adjacent the die aperture; the method which consists in initiating the single stroke and continuing it to the completion of the extrusion within a range of velocities the highest of which yet produces strain hardening within the product and the lowest of which is at least substantially equal to said critical minimum velocity.

5. A method of extruding strain hardenable metal, comprising positioning adjacent to an extrusion die aperture a billet of said metal having a cross-section, length and volume so related to the cross-sectional area of said aperture that extrusion of a major part of the billet will produce an extruded section many feet long at a predetermined extrusion ratio; applying extruding pressure 19 to said billet when the billet is at a temperature so low that plastic deformation of the billet metal would cause the metal to be strain hardened and its resistance to being extruded thereby increased while yet within the die to an extent sufficient to stop the extrusion operation if the metal were deformed at a relatively low velocity below the critical minimum velocity for said particular metal at said predetermined extrusion ratio, said critical minimum velocity being the lowest velocity which when maintained throughout a single extrusion stroke in the extrusion of a billet of a particular metal in strain hardenable condition at a predetermined extrusion ratio will result in continuously expressing the successive plastically deformed portions of the billet from the extrusion die Without causing a substantial rise in resistance to the forcing of the metal out of the die cavity when the total time required to produce the extrusion in a single stroke is greater than the time period from the beginning of plastic deformation to the setting in of strain hardening in each such successive portion of the billet brough adjacent the die aperture; and continuously extruding the billet in a single troke at a substantially uniform pressure and at a substantially uniform velocity at least as high as said critical minimum velocity and yet so low as to avoid impairing the physical condition of the extruded product by heating effects of the die, said substantially uniform extrusion velocity, said predetermined extrusion ratio, the strain hardening properties of said metal and the length of the billet in the direction of extrusion being so related that the metal in the portion of the billet at any time adjacent the die aperture undergoes plastic deformation substantialy greater than any plastic deformation of the billet metal remote from said die aperture, and said substantially uniform extrusion velocity being such that the time elapsing between inception of substantial plastic deformation of each successive portion of the billet brought adjacent to said die aperture and the extrusion of each said successive portion through said aperture is less than the time required for strain hardening to set in in the so plastically deformed billet portion,

the total time required for performing the entire extrusion operation being greater than the time within which such strain hardening of each of said billet portions may set in.

6. A method according to claim 5 in which the billet A metal principally comprises aluminum and the billet is at substantially room temperature when initially compressed, the pressure P exerted on the billet being substantially such that P =Ka(1+e )6 in which P is the loading presure per unit area of die face for forward extrusion or per unit billet area subjected to pressure for backward extrusion, in British tons (2240 pounds) per square inch, a is the stress per unit area, in British tons per square inch, on the true stress-true strain diagram of the metal in compression at the maximum strain value (e obtained in tension, e is the natural or maximum strain, in inches per inch, at the ultimate strength or point of fracture in the tensile test, 5 is the extrusion ratio, i.e. the ratio of the area of the billet to the area of the extruded section, and K is a numerical constant which for regular solid sections is 1.9; for regular tubular sections is 2.1; for irregular solid sections is 2.3; and

20 for irregular hollow sections is 2.5, the lowest velocity V of said range of velocities being substantially such that: i

P =YPe OVR in which P is the loading presure per unit area, in British tons per square inch. W is the yield point in tension or compression of the billet metal, in British tons per square inch as determined by the intersection of the extension of the straight line portion of the plastic deformation curve with the stress axis, e is 2.718 (base of Naperian logarithms), 6 is the extrusion ratio,

a is the true stress in compression in British tons per square inch at maximum strain (e of a tension specimen, and V is the critical minimum velocity in inches per second required to be maintained throughout the extrusion operation.

References Cited in the file of this patent UNITED STATES PATENTS 416,077 Robertson Nov. 26, 1889 1,892,789 Singer Jan. 3, 1933 2,036,182 Singer Mar. 31, 1936 2,063,563 Sparks Dec. 8, 1936 2,218,459 Singer Oct. 15, 1940 2,494,935 Dunn Jan. 17, 1950 2,507,638 Leahy May 16, 1950 2,559,523 Templin July 3, 1951 2,671,559 Rosenkranz Mar. 9, 1954 2,753,994 Bridge July 10, 1956 FOREIGN PATENTS 745,006 Great Britain Feb. 15, 1956 OTHER REFERENCES Metal Flow and the Extrusion Defect, from American Machinist of May 15, 1950, pages 108 to 112.

Extrusion of Aluminum Parts for Douglas Aircraft, from Machinery of July 1945, pages 139 to 147.

The Metallurgy of Deep Drawing and Pressing, by Dudley Jevons, 2nd. edition, published by John Wiley and Sons Inc., 440 Fourth Avenue, New York, N.Y., pages 543 to 551 and 622 to 625.

Tool Engineering Handbook, A.S.T.E. Handbook Committee, 1949 edition, published by McGraW-Hill Book Co., New York, N. Y., pages 467, 468, 470.

Cold Shaping of Steel, by Heintz Manufacturing Co., Philadelphia 20, Pa., issue July 1, 1947, pages 21 to 23.

The Extrusion of Metals, Machinerys Reference Series Number 110, The Industrial Press, 49-55 Lafayette St., New York, N.Y., pages 28 to 34, copyright 1913.

The Extrusion of Metals, by Claude E. Pearson, John Wiley and Sons Inc., 440 4th Avenue, New York, N6.4Y., copyright 1944 and copyright 1953, pages to 1 The Extrusion of Plastics, Rubber and Metals, by Herbert R. Simonds, Archie J. Weith, and William Schack; Reinhold Publishing Corp., 330 West 42nd St., New York, N.Y., copyright 1952, pages 345 to 362. 

1. IN THE PRODUCTION OF A SINGLE EXTRUSION STROKE OF AN EXTRUSION COMPLETE ARTICLE FROM A BILLET OF A GIVEN STRAIN HARDENABLE METAL UNDER TEMPERATURES WHICH CONTINUE THE METAL IN STRAIN HARDENABLE CONDITION THROUGHOUT THE STOKE, WHICH BILLET, IF PLASTICALLY WORKED WITHIN THE DIE CAVITY UNDER THE EXTRUSION RATIO NECESSARY TO COMPLETE THE EXTRUSION OF THE ARTICLE IN THE SINGLE STROKE BY UNDER A VELOCITY SUBSTANTIALLY LESS THAN THE CRITICAL MINIMUM VELOCITY APPLICABLE TO THE METAL WHEN SO EXTRUDED FOR A TIME GREATER THAN THE DELAY TIME BEFORE SETTING IN OF STRAIN HARDENING APPLICABLE THERETO, WILL BECOME CONSEQUENTIALLY STRAIN HARDENED WITHIN THE CAVITY, SAID CRITICAL MINIMUM VELOCITY BEING THE LOWEST VELOCITY WHICH WHEN MAINTAINED THROUGHOUT A SINGLE EXTRUSION STROKE IN THE EXTRUSION OF A BILLET OF A PARTICULAR METAL IN STRAIN HARDENABLE CONDITION AT A PREDETERMINED EXTRUSION RATIO, WILL RESULT IN CONTINUOUSLY EXPRESSING THE SUCCESSIVE PLASTICALLY DEFORMED PORTIONS OF THE BILLET FROM THE EXTRUSION DIE WITHOUT CAUSING A RISE IN RESISTANCE TO FORCING THE METAL OUT OF THE DIE CAVITY WHEN THE TOTAL TIME FOR PERFORMING THE SINGLE STROKE EXTRUSION OPERATION IS GREATER THAN THE TIME DELAY BEFORE SETTING IN OF STRAIN HARDENING OF EACH SUCH SUCCESSIVE PORTION OF THE BILLET BROUGHT ADJACENT THE DIE APERTURE; THE METHOD WHICH CONSISTS IN INITIATING THE SINGLE STROKE AND CONTINUING IT TO THE COMPLETION OF THE EXTRUSION UNDER A SUBSTANTIALLY UNIFORM VELOCITY AT LEAST SUBSTANTIALLY AS HIGH AS AND SO RELATED TO THE SAID CRITICAL MINIMUM VELOCITY THAT THE RESULTING ARTICLE IS HIGHLY STRAIN HARDENED AND ENDOWED WITH SUBSTANITALLY UNIFORM PHYISCAL PROPERTIES IRRESPECTIVE OF THE LENGTH OF THE ARTICLE AND THE TOTAL TIME REQUIRED FOR THE EXTRUSION. 